Hitherto, many proposals have been made on the process for synthesis of methacrylic acid by gas phase catalytic oxidation of methacrolein and on the catalysts used therefor. Especially, since heteropoly-acid type catalysts have been found as catalysts for oxidation of methacrolein, improvement of the catalysts has been proposed in many patents such as, for example, Japanese Patent Kokai (Laid-Open) Nos. 58-96041, 61-7233, 62-153243, 3-238051, etc.
When these catalysts are used in a fixed bed, the catalyst particles must be molded into a suitable size. For the molding of catalysts, it is necessary to employ the conditions most preferable for the properties of the catalyst components per se and the reaction used, and in the case of such a partial oxidation reaction of methacrolein to methacrylic acid as of the present invention, pores inside the molded catalyst are important to obtain the desired product in a high yield. By securing the pores, catalysts excellent in both the activity and the selectivity can be obtained.
However, the heteropoly-acids used in the present invention are inherently poor in moldability and when the conventional pressure molding is employed, a high pressure is required in order to make a molded product having a mechanical strength which can stand industrial use. Therefore, it is difficult to secure the pores in the molded catalysts.
For solving the above problem, mention may be made of the wet molding method. In general, according to the wet molding method, the dry catalyst powder is mixed with an assistant such as a moisturizing component, the resulting mixture is molded into a suitable size, and the molded product is heat treated to remove the moisturizing component. Therefore, this method has the merit that it is easy to secure pores inside the molded catalyst as compared with the pressure molding. Water is normally used as the moisturizing component in the wet molding.
However, since the constituting component of heteropoly-acids dissolves in water, when the wet molding method is applied to the heteropoly-acids and if only water is used as the moisturizing component, there is the tendency that the catalyst active component dissolves away and consequently the inherent performance of the catalyst cannot be exhibited.
As examples of adding a moisturizing component other than water at the time of wet molding, mention may be made of adding lower alcohols such as methyl alcohol and ethyl alcohol as reported in Japanese Patent Kokai (Laid-Open) Nos. 60-48148 and 63-31514. However, these are all supported-type catalysts which comprise a carrier on which a dry catalyst is deposited together with a lower alcohol. Since these supported-type catalysts are smaller in the substantial catalyst amount than molded catalysts prepared by shaping only the catalyst, the reaction temperature must be increased for maintaining a sufficient catalytic activity. As a result, the catalyst life is apt to be adversely affected and further improvement is desired for the use as industrial catalysts.
Furthermore, Japanese Patent Kokai (Laid-Open) No. 1-207137 proposes a process which comprises preparing a concentrated paste containing an alkali metal carbonate, water and an alcohol and extrusion molding the paste. However, this is a process for producing a catalyst carrier used for dimerization of olefins and essentially differs from the method for molding catalyst particles.
Furthermore, in order for controlling the pores inside the molded catalyst, there are reports of adding a polyhydric alcohol such as pyrogallol at the time of preparing the catalyst as in Japanese Patent Kokai (Laid-Open) No. 51-136615 and of adding an organic substance such as polyvinyl alcohol at the time of molding the catalyst, heat treating the molded catalyst and using it as a finished catalyst.
The processes of adding polyhydric alcohols or polyvinyl alcohol suffer from the problems that they lack reproducibility of preparation since decomposition temperatures or removing temperatures of these organic compounds are relatively high and there may occur sintering or reduction of the catalyst due to burning of the organic compounds when they are heat-treated. Thus, further improvement has been desired as a process for the production of industrial catalysts.
Furthermore, since the supported-type catalysts are smaller in substantial catalyst amount than molded catalysts which are prepared by shaping only the catalysts, the reaction temperature must be raised for maintaining a sufficient catalytic activity. As a result, there is the defect that the catalyst life is apt to be adversely affected, and further improvement is desired in the use as industrial catalysts.